1. Field of the Invention
The present invention relates to structural steels which are useful in fabricating pressure vessels, and more particularly it relates to improved chromium-molybdenum type steels which can be formed into thick plates that are weldable together to make chemical pressure vessels.
2. The Prior Art
Pressure vessels used in oil refining installations and coal liquifaction facilities must be capable of withstanding high temperatures and must display a strong resistivity to hydrogen attack. Many pressure vessels must also pass specific water pressure tests, e.g., when they are used as boilers. Some pressure vessels must particularly possess satisfactory low-temperature toughness. One type of commonly used materials for fabricating such pressure vessels is the 21/4Cr-1Mo type steels, which display generally good strength, creep resistivity and hydrogen attack resistivity at elevated temperatures. However, when these steels are exposed to high temperatures (e.g., from 400.degree.-550.degree. C.) over long periods of time, they display heavy embrittlement and lose their toughness. At the same time, growing demand for pressure vessels having enlarged dimensions and which are capable of withstanding increased working pressures has entailed the use of Cr-Mo steel plates having increased thicknesses, e.g., of 100 mm or more. However, when manufacturing steel plates of such thicknesses, the cooling rate for the plates during the normalizing treatment will be reduced, thereby lowering their strengths and toughnesses, as well as limiting the conditions under which stress relief annealing after welding or post weld heat treatment (hereinafter abbreviated as PWHT) can be conducted (after welding) and making it difficult for the steels to retain the necessary strengths subsequent to the PWHT.
Investigations into the effects on temper embrittlement (.DELTA. vTs) resulting from the addition of phosphorus and silicon to 2 to 3Cr-1Mo steel plates of 150 mm in thickness, which plates have been subjected to heat treatments comprising the steps of heating at 940.degree. C. for one hour, normalizing by placement in air, standing at 710.degree. C. for ten hours, and then PWHT, are represented in the graph in the attached FIG. 1. The investigations on temper embrittlement of steel plates of 150 mm in thickness subjected to normalizing at 940.degree. C. for one hour and PWHT at 710.degree. C. for 10 hours, which plates have been produced by rolling the base steels with variation of P and Si contents containing in 0.15C-0.65Mn-2.7Cr-1.0Mo steel at 1100.degree. C. for one hour, are represented in the attached FIG. 1. .DELTA. vTs represents the difference in a fractum transition temperature of V-notch Charpy test (vTs) before and after the step cooling having a heat pattern shown in FIG. 3, the values of which are indicated respectively by the encircled figures in FIG. 1.
The results of the investigations on steel plates of 50 mm in thickness subjected to normalizing at 930.degree. C. for one hour and PWHT at 710.degree. C. for ten hours, which plates have been produced by rolling the base steels with variation of Si content containing in 0.15C-0.50Mn-2.7Cr-1.0Mo steel at 1,100.degree. C. for one hour, are represented in the attached in FIG. 2. It can be seen that in each case embrittlement was improved by lowering the phosphorus and silicon contents. However, it should be remembered that lowering the phosphorus content (for example, to less than 0.010%) cannot be achieved without a special (and costly) processing step, and that lowering the silicon content concurrently results in a loss of strength shown in FIG. 2.
One proposal for solving the noted problems is disclosed in JA-OS No. 41962/1980 (laid open for public inspection on Mar. 25, 1980). In this proposal, the employed Cr-Mo steels have aluminum and boron or boron and titanium added thereto. However, when steel plates having thicknesses exceeding 100 mm are produced from the steels, they are deficient in strength and toughness, i.e., because of a low manganese content. Furthermore, this proposal allows for a wide range of boron content, often exceeding 0.0015%, which indicates that the steel plates suffer from ferrite precipitation during the normalizing treatment and fail to acquire a sufficient strength. Thus, this proposal does not produce steel plates which exhibit sufficient strength, sufficient endurance to high-temperature PWHT, sufficient properties at its heat-affected zones (hereinafter abbreviated as HAZ), or allow for the inclusion of silicon. Also, they employ excess amounts of boron.
An object of the present invention is to provide high tensile strength structural steels which will form thick plates useful in fabricating pressure vessels, which steels will display little tempering embrittlement and low cost.